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No matter how well you seal your decking boards, you're likely to have water problems if the deck doesn't drain properly. Standing water can discolor and warp the wood and promote the growth of rot and fungus. It also creates a slipping hazard. Drainage issues are best addressed when you are installing the decking boards. If you don't notice these problems until afterward, most solutions involve removal and reinstallation of the decking boards as well as possible substructure modifications. To divert drainage from an overhanging deck, you may also have to remove the decking boards.

1

Provide a minimum gap of 1/8 inch between the decking boards. This spacing allows water to run off and prevents it from becoming trapped between the bottom of the boards and the joists and rotting the wood. If the boards have already been installed and are touching, remove them and reinstall them with the proper spacing. Spacing the boards may solve your drainage problems.

2

Provide further drainage by adding furring strips between the joists and decking boards to create a gentle slope from one end of the deck to the other along the lengths of the boards. This modification requires removal of the decking boards if the deck is already assembled.

3

Use a table saw to rip strips that are the same lengths as the joists from 2-inch pressure-treated lumber. If the joists are spaced 24 inches apart, which is standard, make each strip 1/2 inch thicker than the previous one to give the deck a slope of 1/4 inch per foot.

4

Nail or screw the thickest strip to the rim joist at the high end of the deck and the second-thickest strip to the joist next to it. Continue in this way to the other side of the deck, leaving the opposite rim joist without a strip.

5

Install a membrane over the joists to prevent water from dripping from the deck, if desired. A variety of plastic or vinyl membranes are available. Nail it to the tops of the joists and allow it to sag between them. Water will run off in the direction of the slope you have created.

6

Lay the decking boards on the joists, and attach them with 16d galvanized nails or 3-inch decking screws. Be sure to space the decking boards by at least 1/8 inch.

Things You Will Need

Table saw

2-inch pressure-treated lumber

Drainage membrane

Galvanized nails

Decking screws

Tip

Whenever you attach a deck to a building, it's important to provide a way for water to drain off the deck without damaging the siding. You usually do this by installing flashing behind the ledger board or spacers between the ledger board and the siding.

About the Author

Chris Deziel has a bachelor's degree in physics and a master's degree in humanities. Besides having an abiding interest in popular science, Deziel has been active in the building and home design trades since 1975. As a landscape builder, he helped establish two gardening companies.